Current once-through evaporator technology may be employed with large HRSGs to provide two stages of heat exchange. The first stage produces steam/water mixture. The second stage evaporates the water to dryness and superheats the steam. In general, each stage of the HRSG includes a parallel array of heat transfer tubes where internal mass flow rate is controlled by buoyancy forces, and is proportional to the heat input to each individual tube. One type of evaporator uses vertical tubes arranged in a sequential array of individual tube bundles, where each tube bundle (or harp) has a row of tubes that are transverse to the flow of the hot gas. The individual harps are arranged in the direction of gas flow, so that each downstream harp absorbs heat from gas of a lower temperature than the harp immediately upstream. In this way, the heat absorbed by each harp in the direction of gas flow is less than the heat absorbed by the upstream harp. This type of evaporator is similar to that disclosed in U.S. Pat. No. 6,189,491 entitled “Steam Generator”, filed on Jun. 14, 1999, which is incorporated herein by reference.
HRSGs using this principle require the distribution of a water/steam mixture (two-phase flow) from the outlet of a primary evaporator into a secondary evaporator, where dry-out and superheat takes place. The secondary evaporator is formed from one or more harp bundles with multiple inlets on the bottom header. Each inlet provides two-phase flow through a branch connection into the lower header. Each inlet to a header of the secondary evaporator receives two-phase flow from a mixing device downstream of the primary evaporator.
Two-phase flow from one inlet connection is distributed along the length of a portion of the header to outlet tubes in the upper portion of the header. Each outlet tube is an individual evaporator tube in the respective row of the secondary evaporator.
It is known by those skilled in the art that separation of two-phase flow can occur in the bottom header of the secondary evaporator, leading to non-uniform distribution of water/steam mixture into the secondary evaporator heat exchanger tubes within a particular tube row (or harp). For equal mass flow rates, in tubes receiving a higher steam fraction, the water/steam mixture will evaporate to dryness sooner, leading to higher degree of superheat at the exit of the individual tube. In tubes receiving a higher water fraction, the water/steam mixture will evaporate to dryness later, leading to lower degree of superheat at the exit of the individual tube. The thermal expansion of an individual evaporator tube is determined by the integral of the temperature rise of the internal fluid along the length of the tube.
The integrated average temperature of the tube with the higher superheat at the outlet will be higher that the integrated average temperature of tube with lower superheat at the outlet. When adjacent tubes in an individual harp inlet header receive different water/steam fractions, the integrated average of the tube temperature will be different for each tube. Since the tubes are constrained at the upper and lower end by being joined to a common header at both ends, differential temperature in adjacent or nearby tubes will cause a differential thermal stress to develop in the tubes. During startup and load ramps, the non-uniform flow distribution in the inlet headers of the secondary evaporator will vary in location and degree. It has been demonstrated that the location of high differential thermal stress will change during these conditions. An individual tube may transition from a state of no differential thermal stress, to a state of high stress during startup or load ramps. This change of stress has been shown to lead to an alternating stress at the tube joint at the branch connection. When the magnitude of this stress is sufficiently high, and when the number of occurrences reaches a predictable amount, the tube joint is susceptible to failure from low-cycle fatigue.
The evaporator of the present invention applies the principles of an equalizing chamber within the first and/or second stage evaporator to mitigate the effects of the two-phase flow separation at the inlet of the second stage of the evaporator, as will be described in greater detail.